In general, in order to manufacture semiconductor integrated circuits such as ICs and LSIs, various processes, such as a film deposition process, an oxidation and diffusion process, and an etching process, are repeatedly performed to semiconductor wafers. When the respective processes are performed, it is necessary to transfer the semiconductor wafers between apparatuses. In this case, a plurality of, e.g., twenty five semiconductor wafers stored in a storage box are transferred at a time. As the storage box of this type, there are known a cassette and a FOUP (registered trademark) (Patent Documents 1 to 3). The cassette transfers semiconductor wafers, with an inside thereof being opened to an atmospheric air. On the other hand, the FOUP transfers semiconductor wafers stored in a box that is hermetically sealed by a lid, with an inside of the box being filled with an inert gas such as an N2 gas or a clean air, in order to restrain adhesion of particles and a natural oxidation film to the semiconductor wafers.
A processing system of a batch type, for example, which handles the storage box, includes a box transfer area in which the storage box is transferred by a transfer mechanism, and a process-object transport area in which semiconductor wafers are transported so as to be thermally processed, from the storage box to a wafer boat (for example, Patent Documents 4 to 6). Those areas are separated by a partition wall having a gate capable of being opened and closed for sending and receiving wafers. The process-object transport area, in which process objects are transferred in a bare state, has an inert gas atmosphere such as a nitrogen atmosphere, or a clean atmosphere.
In the process-object transport area, a plurality of, e.g., twenty five wafers stored in the storage box are transported at a time to the wafer boat, until the wafer boat becomes completely filled with the wafers, for example. In this case, a certain time period is required to finish the transport operation. For example, when a capacity of the wafer boat allows accommodation of one hundred wafers, it is necessary to transport wafers, while exchanging the four storage boxes. Thus, depending on a performance of a transport arm mechanism, it takes as long as fourteen minutes, for example, for the transport operation.
Thus, in order to improve a throughput, a plurality of, e.g., two wafer boats are generally provided in the process-object transport area. While wafers are thermally processed in one boat, transport of thermally processed wafers and unprocessed wafers is performed in the other boat, so as not to waste time as much as possible (Patent Documents 4 and 5).    [Patent Document 1] JP8-279546A    [Patent Document 2] JP9-306975A    [Patent Document 3] JP11-274267A    [Patent Document 4] JP2002-76089A    [Patent Document 5] JP2003-37148A    [Patent Document 6] JP2004-22674A
In the transport arm mechanism for transporting wafers, there are used components which are relatively susceptible to a heat, i.e., which are poor in heat resistance. For example, the transport arm mechanism is provided with an exposed mapping sensor that detects positions of wafers supported by the wafer boat and positions of wafers stored in the storage box in a tier-like manner. In addition, the transport arm mechanism incorporates various electronic components. Further, a driving force of the transport arm mechanism is often obtained by a compressed air. In this case, there are disposed, in an exposed manner, air pipes through which the compressed air is supplied and discharged. The mapping sensor, the electronic components, the air pipes, and so on, are all poor in heat resistance, and are capable of resisting heat as low as about 50° C. to 200° C., which varies between the members.
In the process-object transport area, although a cooling gas or a clean air is circulated, an atmospheric temperature, in particular an upper atmospheric temperature in the process-object transport area, sometimes exceeds 200° C. by a convection and a radiation heat, when thermally processed wafers, which have a temperature of as high as 700° C. to 800° C., are unloaded downward in the process-object transport area. In order to prevent the transport arm mechanism that is poor in heat resistance from being thermally damaged, the wafer transport operation is forbidden in such a manner that, in the course of unloading the wafers, the transport arm mechanism is withdrawn to a bottom position of the process-object transport area in which a temperature is relatively lower.
In this case, the transport arm mechanism is withdrawn for a long time, namely, from when the unloading operation is started to when the upper atmospheric temperature in the process-object transport area is sufficiently cooled after completion of the unloading operation. There is a possibility that, although the upper atmospheric temperature in the process-object transport area has been lowered to a safety temperature for the respective aforementioned members constituting the transport arm mechanism, the wafer transport operation remains forbidden. This invites deterioration in throughput.